Default Routes and Floating Static Routes

A default route is a static route to "everywhere else." Instead of a specific prefix, it uses the all-zeros network and all-zeros mask:

ip route 0.0.0.0 0.0.0.0 203.0.113.1

0.0.0.0/0 matches every IPv4 destination, because a zero-length prefix imposes no constraint. That makes it the ultimate catch-all: a router with this route can forward a packet for any destination it has no more specific entry for, sending it to the gateway of last resort — here the ISP at 203.0.113.1. This is exactly how a stub site or a home router reaches the whole Internet with one line instead of thousands of specific routes.

The critical word is last. Because route selection uses longest-prefix match, 0.0.0.0/0 — the shortest possible prefix — loses to any route with more matching bits. If R1 has both S* 0.0.0.0/0 and S 8.8.8.0/24, then a packet for 8.8.8.8 takes the /24 (24 matching bits beat 0). The default only wins for destinations that match nothing more specific. So a default route never overrides your other routes; it quietly sits underneath them as the fallback. That answers the baseline: the specific 8.8.8.0/24 wins for 8.8.8.8.

In the table, a default route appears with a distinctive marker:

Gateway of last resort is 203.0.113.1 to network 0.0.0.0
S*   0.0.0.0/0 [1/0] via 203.0.113.1

The * next to S flags it as a candidate default — the route feeding the "gateway of last resort" line. If you ever see traffic to unknown destinations being dropped, the first thing to check is whether that gateway-of-last-resort line is present at all.

Default route as candidate default (topic guide)

A floating static route is an ordinary static route wearing a higher administrative distance so it stays out of the table until it is needed. Administrative distance (AD) is how a router ranks route sources offering the same prefix: lower AD is more trusted, and only the winner is installed into the routing table. A normal static route has AD 1. If you configure a second static route to the same destination but append a larger AD number, IOS keeps it in reserve:

! Primary path (default AD 1) — installed and forwarding
ip route 10.1.4.0 255.255.255.0 10.1.2.2

! Floating backup (AD 5) — held in reserve, not in the table while the primary is up
ip route 10.1.4.0 255.255.255.0 10.1.5.2 5

While the primary is up, only the AD 1 route via 10.1.2.2 is installed — the AD 5 route is not even visible in show ip route, because a less-trusted route for a prefix that already has a winner is held back. The instant the primary route leaves the table — the interface fails, the next hop becomes unreachable, or a dynamic protocol withdraws it — the floating route is the next-best source, so IOS installs it and traffic fails over to 10.1.5.2 automatically. When the primary returns, it reclaims the slot (AD 1 beats AD 5) and the floating route goes dark again.

The number you pick just has to be higher than the primary's AD and lower than any source you want to lose to. A floating static backing up a primary static (AD 1) commonly uses something small like 5. A floating static backing up OSPF (AD 110) would need an AD above 110 to stay dormant while OSPF is healthy. Two things a floating static is not: it is not load balancing (only one path is used at a time), and a second route does not become a backup just by existing — it must carry a higher AD, or both routes install as equal-cost paths.